The invention relates to low-tetroxane aqueous formaldehyde solutions having a pH of from 3.4 to 4.2 comprising
a) from 10 to 75% by weight of formaldehyde
b) from 0 to 500 ppm by weight of tetroxane.
The invention further relates to processes for their preparation and to a process for preparing 1,4-butanediol by reacting the low-tetroxane aqueous formaldehyde solutions with acetylene to form 1,4-butynediol and subsequent hydrogenation of the 1,4-butynediol to form 1,4-butanediol.
Aqueous formaldehyde solutions and processes for their preparation are generally known (cf. Ullmanns Encyclopxc3xa4die der technischen Chemie [Ullmann""s Encyclopedia of Industrial Chemistry], Verlag Chemie, GmbH, Weinheim, 4th edition, volume 11, keyword formaldehyde, chapter 3).
For this purpose, generally, methanol is first dehydrogenated oxidatively with air and the formaldehyde-containing gas stream formed as a result is brought into contact with water in an absorption zone, an aqueous formaldehyde solution being formed.
However, the formaldehyde solutions formed in this process have the disadvantage that the formaldehyde partially reacts even within a very short time to form tetroxane of the formula (I) 
so that, in concentrated formaldehyde solutions, frequently as soon as within one day after their preparation, several hundred ppm of tetroxane form.
Formaldehyde solutions are used as an intermediate in the synthesis of many chemical substances. In many cases here, the tetroxane is not reacted, but remains as residue in the waste water during the work up of the reaction mixture. Since tetroxane is categorized as a hazard to water according to relevant environmental directives, waste waters of this type must be freed from tetroxane by complex clean up operations. This problem arises, in particular, in the preparation of 1,4-butanediol by reacting the aqueous formaldehyde solutions with acetylene to form 1,4-butynediol and subsequent hydrogenation of the 1,4-butynediol to form 1,4-butanediol, the tetroxane being produced in a mixture with water and the product of value 1,4-butanediol.
It is an object of the present invention, therefore, to prepare aqueous formaldehyde solutions which, despite relatively long storage, comprise no, or only small amounts of, tetroxane.
We have found that this object is achieved by aqueous solutions of the type described at the outset, processes for their preparation and a process for preparing 1,4-butanediol using these solutions.
Since formaldehyde solutions are generally handled as solutions having a formaldehyde content of from 30 to 60% by weight, the preferred aqueous solutions according to the invention usually likewise comprise formaldehyde in these concentrations.
The aqueous solutions according to the invention generally comprise from 50 to 400, preferably from 50 to 300, ppm by weight of formic acid and from 0.2 to 5.0, preferably from 0.5 to 2.5, % by weight of methanol, since in the majority of industrial processes for preparing formaldehyde, the formaldehyde is produced in the form of formaldehyde-loaded gas streams which usually comprise
from 5 to 30% by weight of formaldehyde
from 0.5 to 2.0% by weight of methanol
from 30 to 55% by weight of nitrogen
with or without traces of formic acid.
The industrial preparation of formaldehyde and aqueous formaldehyde solutions is generally known and is described, for example, in Ullmanns Encyclopxc3xa4die der technischen Chemie [Ullmann""s Encyclopedia of Industrial Chemistry], Verlag Chemie, GmbH, Weinheim, 4th edition, volume 11, keyword formaldehyde, chapter 3.
In the subsequent preparation of aqueous formaldehyde solutions by selective absorption of these formaldehyde-loaded gas streams in water, it is unavoidable that a certain proportion of methanol and formic acid is co-absorbed. In addition, in the storage of the solutions, methanol and formic acid are formed by disproportionation of formaldehyde. Tetroxane formation in these solutions may be effectively avoided or retarded by adjusting their pH to from 3.4 to 4.2.
This is achieved using Brxc3x6nsted bases such as ammonia, an alkali metal hydroxide or an alkali metal carbonate. Generally, the aqueous formaldehyde solutions comprise from 0.001 to 0.1% by weight of the Brxc3x6nsted base, based on the formaldehyde absorbed therein.
Expediently, a procedure is followed such that
I. a formaldehyde-loaded gas stream is absorbed in water and
II. in the course of 30 min after the absorption, from 0.001 to 0.1% by weight of a Brxc3x6nsted base, based on the formaldehyde absorbed, is dissolved.
Particularly expediently, to prepare the solutions according to the invention, a procedure is followed such that the base is added as early as immediately after or during the preparation of the conventional formaldehyde solutions. This can be performed industrially, for example, by absorbing, in a plurality of absorption stages, in water or aqueous formaldehyde solutions, a reaction gas, such as is produced in the oxidative dehydrogenation of methanol by air on a silver catalyst, and adding the base to the absorption stage to which the reaction gas is fed directly.